In offshore petroleum operations, platforms comprising a jacket secured to the seafloor and a deck mounted on top of the jacket are commonly used to drill for and produce oil and gas. Typically, the deck is mated to the jacket after the jacket has been installed. This is usually accomplished by lifting individual components of the deck, including deck sections, crew facilities, and drilling and production equipment, onto the jacket with a barge-mounted crane. After the individual components are lifted onto the jacket, they are integrated together.
This approach generally works quite well, but costs can be very high due to the offshore construction required. Offshore construction is very expensive for a number of reasons, including down-time caused by rough weather and the need for special offshore construction vessels. In the case of very large platforms or platforms located in remote areas, offshore construction may require as much as one million manhours and ten months to complete.
There is another approach to mating platform decks and jackets, called the integrated deck approach, which has been introduced in recent years. With the integrated deck approach, a one-piece deck is used, with most or all components being integrated together at an onshore construction yard. By using an integrated deck, offshore construction time is greatly reduced. This not only substantially reduces offshore construction costs, it also makes the approach attractive for offshore areas having short construction seasons due to rough seas or due to the presence of sea ice.
Because an integrated deck consists of a single unit comprising most or all of the components used for drilling and production, it is very heavy. For this reason, integrated decks are not lifted onto platform jackets with barge-mounted cranes. Instead, the integrated deck is carried on a barge to the jacket, and the barge is then ballasted to lower the integrated deck onto the jacket. Typically, the jacket will have a slot into which the barge is maneuvered. The integrated deck extends over both sides of the barge and mates with the jacket as the barge is ballasted downward.
Since the integrated deck is carried by a barge during the mating operation, it is subject to movement caused by the action of wind on the barge and deck, and more importantly, by the action of waves on the barge. This movement can make proper alignment of the integrated deck with the jacket very difficult. Although various apparatus for aligning integrated decks with jackets have been used and proposed, these apparatus are generally not satisfactory for use in seas exceeding one or two feet, or are too complicated and expensive to be practical. Thus, the integrated deck approach is currently limited to areas where higher seas are not likely during the mating operation. Thus, the advantages offered by the integrated deck approach currently cannot be realized to the extent desired by the petroleum industry. For this reason, there is a need for a practical apparatus which can permit the alignment of an integrated deck with a jacket in higher seas. The present invention is aimed at providing such an apparatus.